(1) Field of the Invention
The present invention relates to a heater control device that controls a heater in which an intensive rush current arises, and more particularly relates to a heater control device which is suitable for an image forming apparatus such as a copier, laser beam printer, facsimile machine etc., in which a halogen lamp is used for the fixing heater.
(2) Description of the Prior Art
Referring to FIGS. 1 and 2, a conventional example will be explained.
In conventional image forming apparatuses such as copiers, laser beam printers, facsimile machines, etc., a fixing heater 1 using a halogen lamp etc. which generates intensive rush current is most commonly adapted to be activated at the zero-cross point, in order to reduce the noise flowing out from the a.c. cable to the power supply line as much as possible.
Even with the above countermeasure, fixing heater 1 in a circuit configuration as shown in FIG. 1 has very low resistivity value when the filament of the fixing heater remains cold, so that a very large rush current occurs at the moment fixing heater 1 is activated. In order to suppress the rush current this configuration needs a triac TD, which is rather expensive and also suffers from a problem that large variation of the power source voltage occurs.
To take countermeasures against the aforementioned variations of the power source voltage, there is a method by which fixing heater 1 is activated by gradually making the phase angle of current flow greater, using a reactor L in series between fixing heater and triac TD as shown in FIG. 2. This method, however, includes the problem of occurrence of a large higher-harmonic current because a large amount of current flows through fixing heater 1 during the control of the phase angle of the current through fixing heater 1.
The variation of the power source voltage causes fluctuation, and the higher-harmonic current affects the operations of other appliances by way of the power system. Therefore, in Europe, for example, the international standard (IEC1000-3-2, IEC1000-3-3) concerning EMC has been laid down by force by the IEC since Jan. 1, 1996. Under this rule, new regulations concerning fluctuation and higher harmonics will be started from 1998.
Now, reference will be made to the EC's regulations against higher harmonics and flicker of fluctuation.
First, the regulation for higher harmonics is to regulate higher-harmonic components of the input current to the appliances connected to the power supply, in order to prevent disorders as follows. More specifically, when an electronic appliance is connected to the power system, higher-harmonic current occurs in the power source due to the components whose input power source is not of a sinusoidal current, such as the power source rectifier circuit, phase control circuit, components having non-linear load characteristics and the like, thus causing voltage distortion. This voltage distortion, in turn, affects power system equipment as well as the operations of the appliances connected to the power system, thus causing overheating, burnout and/or generation of noise in the capacitors, inductors and transformers, malfunctions of the control devices, operation failures and shortening of the lifetime of the connected appliances, and other defects.
Secondly, the regulation against flicker is the one which prevents flickering of illumination due to variations of the power supply voltage occurring with the changes in the consumed current of appliances being connected to the power source, and regulates the magnitude of the voltage drop, its duration and frequency of changes per min. when an appliance is operated under the power source impedance as the reference. In this point of view, in the case where an appliance such as a copier employing a halogen lamp of high power as the fixing heater is used, the rush current when the fixing heater is activated, is of major influence, therefore, this rush current needs to be reduced.
As a countermeasure against this, it is disclosed in Japanese Patent Application Laid-open Hei 3 No. 266,008, for example, that the outstanding large rush current which will occur when the fixing heater is activated is prevented by providing a temperature detecting means in the fixing heater, and turning on the thyristor at a zero-cross point in the a.c. input voltage waveform when the detected temperature is low while, if the detecting temperature is high, activating the thyristor at a phase after and around the peak of the a.c. input voltage waveform.
However, the fixing heater using a halogen lamp etc., will cause a rush current every time the heater is activated, this needs use of a control device such as triac etc., having a rated current greater than that required for the control, resulting in increase in cost. Further, there is a concern that this large rush current might cause fluctuation such as flicker of fluorescent lamps connected to the same power system as the copier.
To solve this problem, there is a method of performing a soft start of the activation of the fixing heater by performing phase angle control. This method, however, is accompanied with a large higher-harmonic current, needing a reactor which must be connected in series with the fixing heater. The size of the reactor is determined by its inductance value and average current (the amount of heat generation). Therefore, if it is used with a halogen lamp etc. which has a high current load, the reactor needed becomes larger and heavier and more expensive, hindering the reduction in size and cost of the copier.